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It's one of the lesser-known and least discussed military conflicts in United States history, and yet the Mexican-American War (1846–1848) played a crucial role in changing the nation's boundaries and character. The effect on Mexico, however, was disastrous, the negative impact of the country both immediate and long-lasting. This short but brutal confrontation also reinforced the worst stereotypes each country held about the other. So, what prompted these two neighbors to take up arms against one another, and what was the outcome?

Click through and relive the first US armed conflict chiefly fought on foreign soil.

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On December 27, 1845, US President James K. Polk signed into law a document known as the annexation bill. His signature formally recognized Texas as the 28th state of the Union.

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However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the US asserting it to be the Rio Grande, and Mexico claiming it to be the more northern Nueces River.

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Texas had already gained its independence from Mexico in 1836, established as a result of the Texas Revolution, during which the famous Battle of the Alamo had taken place.

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In the wake of the revolution, the Republic of Texas was born on March 2, 1836. Effectively a sovereign state, it shared borders with the United States and Mexico.

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By 1844, calls to unite Texas with the United States had grown more vocal, not least from Polk's predecessor, President John Taylor. On February 27, 1845, the Senate approved Texas statehood; the House of Representatives approved the bill on the following day. But there was a problem.

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Mexico refused to recognize Texas as legitimate American territory. Furthermore, Texas admission to the United States antagonized both Mexican officials and citizens.

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But what really irked Mexico was the boundary issue, the disputed zone between the Rio Grande and Nueces River. The US government had offered to purchase the land, a pretext seen as an opportunity to also grab California and everything else in between. The offer was declined. Things came to a head when President Polk ordered American forces into the contested territory.

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The disputed territory in fact constituted an area that both countries had previously recognized as part of the Mexican state of Coahuila. Polk's actions were therefore regarded as deliberately provocative, and Mexico reacted accordingly. On April 25, 1846, Mexican cavalry attacked American military forces led by General Zachery Taylor (pictured), killing around a dozen men.

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The Mexicans then laid siege to Fort Texas along the Rio Grande. Taylor called in reinforcements and met with General Mariano Arista, commander of the Mexican forces, on land near present-day Brownsville. The Battle of Paulo ensued—the opening engagement of the Mexican-American War.

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The Battle of Palo Alto and another engagement the following day, the Battle Resaca de la Palma, both resulted in US victories. Taylor crossed the Rio Grande and began his series of battles in Mexican territory. Polk, meanwhile, citing the fact that Mexico had "invaded our territory and shed American blood on American soil," announced that the two nations were now at war. On May 13, 1846, Congress approved the declaration of war.

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During the conquest of California, American forces captured the town of Monterey virtually unopposed. San Francisco Bay was seized, and several other California ports were overrun. By August 1846, the Mexican government in California had ceased to exist.

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The losses Mexico had suffered in just a few short months prompted the return from exile in Cuba of Antonio López de Santa Anna. A name synonymous with the fall of the Alamo, Santa Anna had convinced President Polk that if allowed to return to Mexico as a civilian he would end the war on terms agreeable to the United States.

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Santa Anna, however, fell back on his word. Back on home soil, the general immediately changed into his uniform, took charge of the Mexican Army, and led it straight into combat at the Battle of Buena Vista. Fought over February 22–23, 1847, the battle resulted in Mexican forces suffering heavy casualties to the point where it was forced to withdraw. In compensation, Santa Anna, licking his wounds, assumed the presidency the following month.

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One of the most protracted engagements of the Mexican-American War was the Battle of Veracruz. Veracruz was a key seaport, and if captured would provide American forces with a strategic toehold on the Gulf of Mexico.

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United States troops led by General Winfield Scott conducted an amphibious assault on Veracruz on March 9, 1847. After encircling the city, a 20-day siege ensued. On March 29, the beleaguered Mexicans surrendered. The defeat opened the road to Mexico City.

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Flushed with victory, General Scott marched inland towards the Mexican capital. His advance was largely unhindered, though the two sides clashed at the Battle of Cerro Gordo.

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The Battle of Cerro Gordo took place on April 18, 1847. Despite being outnumbered by Santa Anna's army, US troops were able to outflank the Mexicans, but were surprised and most likely impressed by their enemy's fierce resistance.

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Two more engagements took place as the Americans encroached upon Mexico City—the Battle of Contreras and the Battle of Churubusco. These marked some of the final encounters of the war, with Mexican forces now visibly buckling under the relentless onslaught of American cannon and musket fire.

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On September 13, 1847, American forces reached the outskirts of their objective. But one obstacle remained: Chapultepec Castle.

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Santa Anna and his army of 25,000 held the strategically important castle, located just outside Mexico City. From around 7,200 US troops, General Scott sent 2,000 men to take the stronghold.

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During the battle, six Mexican military cadets—the so-called niños héroes (boy heroes)—purportedly took their own lives rather than surrender. This single act of heroism is today celebrated in Mexico as a civic holiday to honor the cadets' sacrifice.

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The fall of Chapultepec Castle allowed General Scott and his troops to enter the city, which they did so triumphantly on September 14, 1847. For all intents and purposes, the Mexican-American War had ended.

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With Santa Anna having resigned the presidency and after US authorities had waited for a new government to form, the Treaty of Guadalupe Hidalgo was signed on February 2, 1848. The Mexican-American War was officially over.

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Under the treaty, Mexico lost 55% of its territory, including the present-day states of California, Nevada, Utah, most of Colorado, New Mexico, and Arizona, and a small portion of Wyoming. It also established once and for all the Rio Grande (and not the Nueces River) as the US-Mexican border. And Mexico also recognized the US annexation of Texas—the contentious issue that helped provoke the conflict in the first place.

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US casualties during the Mexican-American War totaled about 17,000 (13,000 dead and 4,000 wounded). Mexican casualties during the conflict numbered about 25,000, including possibly 1,000 civilians. Disease accounted for many deaths. In fact, in terms of disease the war was the deadliest of all American wars: nearly 13% of the entire US force perished from infectious diseases, specifically dysentery. As for the financial cost, the United States is estimated to have spent US$100 million. That's a staggering $3.8 billion in today's figures.

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Zachary Taylor, meanwhile, emerged as a national hero and succeeded Polk as president in 1849.

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While most Americans rallied around the flag, there was opposition to the Mexican-American War. Abraham Lincoln, then a congressman, voiced doubt on whether the war had begun on American soil. Ulysses S. Grant called the conflict "one of the most unjust ever waged by a stronger against a weaker nation."

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According to eyewitness accounts, assaults, looting, and violence toward civilians and surrendered Mexican forces were common. Racist attitudes and anti-Catholic bigotry enabled American combatants to justify extreme violence, acts of brutality that today would be regarded as war crimes against Mexican forces and civilians.

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The war reopened the slavery-extension issue. Anti-slavery elements fought for the exclusion of slavery from any territory absorbed by the US. Others disagreed. The issue became a political hot potato, provoking fierce debate in Washington.

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And it was within this social and political context that many officers and soldiers who had fought in the Mexican-American later found themselves taking up arms again, this time against each other in the American Civil War.

Sources: (National Park Service) (History) (The Alamo) (National Center for Biotechnology Information) (Encyclopedia.com) (Official Data Foundation) (Britannica)